In an effort to quantify the risk for late progression in burst fractures, the sagittal index (SI) was defined to help to assess the segmental deformity at the level of the fracture. The SI is a measurement of the kyphotic segmental deformity corrected for the normal sagittal contour at the level of the deformed segment. A prospective study was devised in 1986 for burst fracture treatment. Complete data were available on 35 patients (22 males, 13 females), with an average follow-up of 27 months. SI was greater than 15 degrees in the first group, the members of which were treated surgically. The technique is described. SI was less than 15 degrees in the third group, the members of which were treated conservatively. The second control group included patients with SI greater than 15 degrees but who were not treated according to the authors protocol for independent reasons. The results suggest that SI is a useful criteria to assess deformity and predict progression of segmental kyphosis.